In recent years, a technology has been developed that prevents leakage of confidential information recorded on a medium such as paper. Particular examples of the technology that have been proposed are a technique for embedding encoded information into an image, and a technique for encrypting in advance a digitalized image that cannot be seen by the general public and then printing the encrypted image on a medium (for example, refer to Japanese Laid-open Patent Publication No. H07-254037 and Japanese Laid-open Patent Publication No. 2008-301044). For example, according to the technique disclosed in Japanese Laid-open Patent Publication No. H07-254037, a two-dimensional code is provided including position specifying symbols, a data region, timing cells, and a vertex detection cell that are arranged two-dimensionally according to a predetermined arrangement order.
In the technique disclosed in Japanese Laid-open Patent Publication No. 2008-301044, an encryption apparatus rearranges on a block-by-block basis, pixels in an encryption target region of an inputted image according to a predetermined encryption key. The encryption apparatus also adds a position specifying marker for specifying the encrypted region, to each of at least two of the four corners of the encrypted region. Further, the encryption apparatus adds a check marker for checking the validity of a decrypted image to be produced by decrypting the encrypted region. A decryption apparatus of this technique digitalizes the image including the encrypted region by reading the medium on which the image is printed, by using a reader such as a scanner or a digital camera. The decryption apparatus then decrypts the encrypted region by referring to the position specifying markers in the digitalized image, and thereby obtains the source image.
When an encrypted image produced by using such a technique for encrypting or encoding part of the information of the source image is printed on a medium, it is preferable that the encrypted image be readable by commonly-used readers such as scanners and cameras integrated in mobile phones. In decrypting the encrypted image read by a reader and then re-digitalized, it is preferable that the position of each block be detectable in the re-digitalized encrypted image. To enable this, techniques for adding, to an encrypted image, information for specifying the position of each block have been proposed (for example, refer to Published Japanese Translation of PCT International Publication for Patent Application (Kohyo) No. H09-504660 and Japanese Laid-open Patent Publication No. 2009-232129).
For example, the technique disclosed in Published Japanese Translation of PCT International Publication for Patent Application (Kohyo) No. H09-504660, proposes that reference marks are included in a document that are used for at least one operation including registration, scaling, rotation, shifting, and defect compensation in descrambling.
In addition, according to the technique disclosed in Japanese Laid-open Patent Publication No. 2009-232129, an image processing apparatus divides an image into a plurality of small regions, rearranges the small regions, and then converts the values of pixels constituting a part of each small region. This conversion facilitates detection of the position of each small region.
In the above known techniques, the position of each block is made detectable by inverting or shifting the values of the pixels in particular blocks, or inverting or shifting the values of at least some of the pixels in each block. However, when an image is compressed according to an image compression standard, such as JPEG, or as for the original values of the pixels in an encrypted image are converted by scanning a printed image, it may be difficult for the decryption apparatus to decrypt an encrypted image to reproduce the original values of the pixels. As a result, the image quality of the decrypted image reproduced by decrypting the encrypted image is degraded. Thus, an image obtained through decryption is likely to have lower image quality when block-position specifying information is added to an encrypted image by converting the values of pixels. For this reason, it is preferable to prevent position specifying information from being added if possible, or to add such information only to part of each block. When position specifying information is not added to an encrypted image, on the other hand, it is difficult for the decryption apparatus to detect the accurate position of each block, which also results in the problem that the image quality of the image produced by decryption is degraded.